1. Field of the Invention
The invention concerns a magnetic resonance system of the type having a whole body arrangement for receiving RF magnetic resonance (MR) signals.
2. Description of the Prior Art
Magnetic resonance systems are generally known that include a data acquisition unit that has an examination region that is open axially on both sides relative to a central axis thereof, the examination region being radially surrounded by an inner wall. A patient on a transport bed is movable into and out of the examination region, along the central axis of the examination region. A substantially homogenous static basic magnetic field is generated in the examination region by a basic field magnet in the data acquisition unit. Radio-frequency energy is radiated into the examination subject by a transmission arrangement that radially surrounds the examination region. The radiation radio-frequency energy causes magnetic resonance signals to be emitted from the examination subject, which are detected by a reception arrangement that also radially surrounds the examination region.
The RF transmission and reception arrangement that is built into the data acquisition unit is referred to as whole-body antenna arrangement, and is stationary with respect to the data acquisition unit.
In known systems of this type, the transmission arrangement is normally identical with the reception arrangement. This combined transmission/reception arrangement is often fashioned as a birdcage resonator.
A homogeneous excitation of the person located in the examination region to magnetic resonances is possible by means of magnetic resonance systems fashioned in such a way. A homogeneous reception of excited magnetic resonances from the entire examination region is also possible. Various three-dimensional reconstructions are determined using the acquired magnetic resonance signals with the use of the whole-body reception arrangement, but only at inferior quality. Therefore local coils are often used for the acquisition of magnetic resonance signals. Qualitatively significantly higher-grade reconstructions are often possible by means of local coils. However, local coils exhibit the disadvantage that they must be manually applied on the patient and also must be manually removed again. Their use is therefore relatively time-consuming. Furthermore, acquisition of magnetic resonance signals by means of an individual local coil is possible only from a small part of the entire examination region. The person must therefore be covered by means of many local coils over a large area. This is often subjectively perceived to be uncomfortable.
The signal strength that occurs in the acquisition mode in magnetic resonance applications is also relatively low. Significant efforts are therefore undertaken to keep the noise optimally low, thus to maximize the signal-to-noise ratio (SNR). The use of cooled local coils is one possibility for minimization of noise. A further possibility is the use of superconducting coils. Superconducting coils are, for example, described in WO-A-01/94964 as well as in the following technical articles:                “Superconducting RF Coils for Clinical MR Imaging at Low Field” by Q. Y. Ma et al., Academic Radiology, Vol. 10, Nr. 9, September 2003, pages 978 to 987;        “Superconducting and Cold Copper MRI Coils” by L. C. Bourne, appearing in ISMRM 5th (1997), page 1527;        “Superconducting Coil Array for Parallel Imaging” by J. Wosik et al., appearing in Proc. Intl. Soc. Mag. Reson. Med. 13 (2005), page 678;        “High Temperature Superconducting Surface Coils with Liquid Nitrogen or Pulse Tube Refrigeration” by Markus Vester et al., appearing in ISMRM 5th (1997), page 1528;        “Superconducting MR Surface Coils for Human Imaging” by Q. Y. Ma et al., placed on the Internet and retrievable at http://www.supertron.com/Product/Publications/pub-3.htm.        
In all publications cited above, small coils are always used as the coils. In one of the publications it is even explicitly stated that a noteworthy improvement of the SNR is to be expected only for coil diameters of at maximum 12 cm. Such dimensions are thus significantly smaller than the typical diameter of a whole-body transmission and reception arrangement. This diameter is normally 50 to 65 cm.
A system of superconducting resonators for magnetic resonance applications is known from EP 1 626 286 A1. The resonators can effect a current distribution that is nearly identical with that of a conventional birdcage resonator. The system can be dimensioned such that magnetic resonance whole-body measurements are possible. It is used both in the transmission case and in the reception case.